1. Field of the Invention
The present invention relates to a fabrication method of semiconductor device, and more particularly, to a fabrication method of a gate electrode of a low-noise compound semiconductor device which is operated at the microwave or millimeter wave.
2. Description of the Conventional Art
Low-noise devices are used in low-noise amplifier for amplifying weak signals in the high-frequency radio communication. The noise properties can be quantificated into the noise figure (NF). Generally, the minimum noise figure NF.sub.min is expressed by the following formula: ##EQU1## It can be seen from the above formula that significant improvements in low-noise device performance can be made by reducing the gate length to 0.5 .mu.m or less in order to reduce C.sub.gs and increasing the transconductance g.sub.m0. But, it was difficult to reduce the noise figure when the gate length is reduced, because the gate resistance R.sub.g is increased according to the reduction of the gate length. And, it was difficult to form a gate having the gate length of 0.5 .mu.m or less by using conventional photolithography technique.
In order to overcome the above-mentioned problem, it was known that a T-shaped gate having a T-shaped cross section is formed by using an electron beam lithography or dielectric materials.
The problems of the prior art will be described with reference to FIGS. 1 and 2.
FIG. 1 is a sectional view of T-shaped gate formed by a conventional E-beam lithography technique. First of all, a first photoresist film 104 is formed and a second photoresist film 106 having a degree of sensitivity to E-beam exposure corresponding to ten times or more of that of the first photoresist film 104 is deposited on the first photoresist film 104. Then, a third photoresist thin film 108, which is composed of the same material as the first photoresist film 104, is formed. When the resultant structure is exposed to E-beam and developed, a relatively wide opening is formed in the second photoresist film 106, and a relatively small opening is formed in the first photoresist film 104 corresponding to the size of the exposed region to E-beam, thereby resulting an opening shaped like a wine goblet. As a result of the development as above, The third photoresist thin film 108 has an opening having an opening size larger than that of the opening in the first photoresist film 104 and smaller than that of the opening in the second photoresist film 106, since the third photoresist thin film 108 suffers a loss at the edge portion during the development. Therefore, it is possible to perform lift-off process for removing unwanted metals but leaving a gate 107. After the patterning step as above, a metal film is formed and lift-off of the remaining metal is performed, thereby forming a T-shaped gate 107.
FIG. 2 is a sectional view of T-shaped gate formed by a conventional photolithography technique using dielectric materials. In this case, instead of the triple photoresist film used in the electron beam lithography technique as described above, a dielectric film 209 is used for defining a gate length. After defining the gate length, a pattern for lift-off having a width greater than the gate length is formed. A T-shaped gate 207 is formed by depositing a gate metal on the resultant structure and performing the lift-off process.
The conventional electron beam lithography technique described above has an advantage of obtaining a gate length of 0.5 .mu.m or less. But, the T-shaped gate according to the conventional technique has a thickness not more than the thickness of the second photoresist film. Further, an expensive equipment is required in the conventional E-beam method while a low productivity is resulted from the conventional E-beam method. Thus, the conventional E-beam method has the problem of a high cost of the production.
The conventional technique using the dielectric materials has an advantage of forming the T-shaped gate by a simple process at a low cost. But, according to the conventional technique using the dielectric materials, the gate length is determined by the etched amount out of the dielectric film. And, a gate length of 0.5 .mu.m or less can not be obtained by a usual photolithography. Further, the thickness of a metal film should be smaller than that of the photoresist film. Thus, the conventional technique using the dielectric materials is limited in decreasing the gate resistance.